1. Technical Field
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave oscillator having the surface acoustic wave resonator, and an electronic apparatus, and more particularly, to a surface acoustic wave resonator in which grooves are formed on a substrate surface, a surface acoustic wave oscillator having the surface acoustic wave resonator, and an electronic apparatus.
2. Related Art
In a surface acoustic wave (SAW) device (such as an SAW resonator), variation in a frequency temperature characteristic is greatly affected by a stop band of the SAW or a cut angle of a quartz substrate, the shape of an IDT (Interdigital Transducer), and the like.
For example, JP-A-11-214958 discloses a configuration for exciting an upper mode and a lower mode of a stop band of an SAW, a standing wave distribution in the upper mode and the lower mode of the stop band, and the like.
JP-A-2006-148622, JP-A-2007-208871, JP-A-2007-267033, and JP-A-2002-100959 disclose that an upper mode of a stop band of an SAW has a frequency temperature characteristic superior than that in a lower mode of the stop band. JP-A-2006-148622 and JP-A-2007-208871 disclose that a cut angle of a quartz substrate is adjusted and a normalized thickness (H/λ) of an electrode is increased to about 0.1 so as to obtain an excellent frequency temperature characteristic in an SAW device using Rayleigh waves.
JP-A-2007-267033 discloses that a cut angle of a quartz substrate is adjusted and a normalized thickness (H/λ) of an electrode is increased to about 0.045 or greater in an SAW device using Rayleigh waves.
JP-A-2002-100959 discloses that a rotational Y-cut X-propagation quartz substrate is employed and that the frequency temperature characteristic is improved, compared with a case where resonance in a lower end of a stop band is used, by using resonance in an upper end of the stop band.
In an SAW device employing an ST-cut quartz substrate, grooves are disposed between electrode fingers of an IDT or between conductor strips of a reflector, which is disclosed in JP-A-57-5418 and “Manufacturing Conditions and Characteristics of Groove-type SAW Resonators”, Technological Research Report of the Institute of Electronics and Communication Engineers of Japan MW82-59 (1982). The “Manufacturing Conditions and Characteristics of Groove type SAW Resonators” also discloses that a frequency temperature characteristic varies depending on the depth of the grooves.
Japanese Patent No. 3851336 discloses that a configuration for setting a curve representing a frequency temperature characteristic to a three dimensional curve is used in an SAW device employing an LST-cut quartz substrate and that any substrate with a cut angle having a temperature characteristic represented by a three dimensional curve could not be discovered in an SAW device employing Rayleigh waves.
As described above, there exist a variety of factors for improving the frequency temperature characteristic. Particularly, in the SAW device employing the Rayleigh waves, increase in the thickness of an electrode which forms an IDT is considered as one of factors contributing to the frequency temperature characteristic. However, the present applicant experimentally found that an environment resistance characteristic such as a temporal variation characteristic or a temperature impact resistance characteristic is deteriorated by increasing the thickness of the electrode. Further, in a case where improvement in the frequency temperature characteristic is a main purpose, the thickness of the electrode should be increased as described above, and it is thus difficult to avoid the deterioration in the temporal variation characteristic, the temperature impact resistance characteristic or the like. This is true of a Q value, and thus, it is difficult to increase the Q value without increasing the thickness of the electrode.